The present invention relates to a process of purifying exhaust gases by removing pollutants from exhaust gases with the use of wet scrubbing and reheating of the exhaust gases before they are discharged into the environment.
Processes of the above mentioned general type are known in the art. Such processes are much more effective than so-called dry or semidry processes of purifying exhaust gas as far as the separation rate is concerned but they require a treatment of the pollutant-containing scrubbing liquor and in most cases require also a reheating of the exhaust gases in order to avoid corrosion or even to avoid a formation of plumes by the exhaust gases as they enter the environment. A formation of a visible plume by the exhaust gas is often regarded as an indication of an excessive emission and for this reason a suppression of such plumes may even be required if this would not be necessary from the aspect of the pollution of the environment. Particularly at locations in densely populated regions it is not always sufficient to purify the exhaust gases by a process which consistently results in pollutant emissions which are lower than all permissible limits but it is also necessary to prevent under all weather conditions which may be expected the occurrence of the innocuous and inevitable emission of water vapor in a visible form.
That requirement can be defined in the known i,x graph of Mollier for humid air in that the state variables of the exhaust gas as it enters the environment and the varying state variables of the mixture consisting of the exhaust gas and the ambient air must always lie above the saturation curve. As the exhaust gases mix with the ambient air, the state variables will vary along a straight line, which is defined in the i,x graph by the values for the exhaust gas, on the one hand, and the values for the ambient air, on the other hand. If that straight line is entirely above the saturation curve, there will be no condensation of water vapor so that an exhaust gas plume will not be visible.
From the above, the requirement can be geometrically derived that with respect to a given point for the ambient air the state variables of the exhaust gas entering the environment must be so selected that they do not lie below a tangent from the predetermined point for the ambient air to the saturation curve. For this reason it is readily apparent that the temperature to which the exhaust gases must be reheated will be the higher the lower is the temperature of the ambient air. This means that the desired suppression of a formation of plumes may require under certain circumstances a considerable expenditure of energy so that the purification of exhaust gases by processes of said kind may be rejected owing to a lack of economy.